Telecommunications apparatus and methods

ABSTRACT

A wireless telecommunications system including first and second terminal devices operable to communicate with one another in a synchronized device-to-device manner. The second terminal device is operable to selectively achieve synchronization for receiving data transmissions from the first terminal device using direct synchronization signalling transmitted by the first terminal device or indirect synchronization signalling transmitted by another network entity, such as a base station. The second terminal device is configured to preferentially seek indirect synchronization over direct synchronization, unless the second terminal device detects synchronization signalling transmitted by the first terminal device is associated with a predefined characteristic that indicates the synchronization signalling transmitted by the first terminal device is high priority synchronization signalling, in which case the second terminal device is configured to preferentially seek synchronization using the high priority direct synchronization signalling from the first terminal device over indirect synchronization signalling transmitted by the other network entity.

BACKGROUND Field

The present disclosure relates to telecommunications apparatus andmethods.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Mobile telecommunication systems, such as those based on the 3GPPdefined UMTS and Long Term Evolution (LTE) and Long Term EvolutionAdvanced (LTE-A) architecture, are able to support more sophisticatedservices than simple voice and messaging services offered by previousgenerations of mobile telecommunication systems. For example, with theimproved radio interface and enhanced data rates provided by LTEsystems, a user is able to enjoy high data rate applications such asvideo streaming and video conferencing on mobile communications devicesthat would previously only have been available via a fixed line dataconnection.

The demand to deploy fourth and higher generation networks is thereforestrong and the coverage area of these networks, i.e. geographiclocations where access to the networks is possible, is expected toincrease rapidly. However, although the coverage and capacity of fourthand higher generation networks may significantly exceed those ofprevious generations of communications networks, there are stilllimitations on network capacity and the geographical areas that can beserved by such networks. These limitations may, for example, beparticularly relevant in situations in which there is a desire for agroup of terminal devices (communications devices) to exchangeinformation with each other in a fast and reliable manner. In order tohelp address these limitations there have been proposed approaches inwhich terminal devices within a wireless telecommunications system maybe configured to communicate data directly with one another without someor all their communications passing through an infrastructure equipmentelement, such as a base station. Such communications are commonlyreferred to generally as a device-to-device (D2D) communications orproximity services (Prose). Many device-to-device communications may betransmitted by one device to a plurality of other devices in a broadcastlike manner and so in that sense the phrase “device-to-devicecommunications” also includes “device-to-devices communications”.

Thus, D2D communications allow communications devices that are insufficiently close proximity to directly communicate with each other,both when within the coverage area of a network and when outside anetwork's coverage area (e.g. due to geographic restrictions on anetwork's extent or because the network has failed or is in effectunavailable to a terminal device because the network is overloaded). D2Dcommunications can allow user data to be more efficiently and quicklycommunicated between communications devices by obviating the need foruser data to be relayed by a network entity such as a base station. D2Dcommunications also allow communications devices to communicate with oneanother even when one or both devices may not be within the reliablecoverage area of a network. The ability for communications devices tooperate both inside and outside of coverage areas makes wirelesstelecommunications systems that incorporate D2D capabilities well suitedto applications such as public protection/safety and disaster relief(PPDR), for example. PPDR related communications may benefit from a highdegree of robustness whereby devices can continue to communicate withone another in congested networks and when outside a coverage area. 3GPPhas developed some proposals for such public safety D2D use in LTEnetworks in Release12.

The automotive industry has been working for several years on solutionsto enable communication with and between vehicles, e.g. to help improvetraffic flow and safety. These techniques can range from automatictolling technologies to collision prevention mechanisms, and aregenerally known as Intelligent Transport Systems (ITS). Currently, themain radio technology under consideration in standards projects relatingto ITS is a WLAN derivative 802.11p, which would be used forbroadcasting ITS information by vehicles or road side infrastructure toother vehicles. This constitutes so-called Dedicated Short RangeCommunication (DSRC) system that is deployed at 5.9 GHz ITS band inEurope and North America (there may be different ITS bands in use inother regions, e.g. 700 MHz in Japan).

The effective range of DSRC systems is a few hundred meters and theservices are broadcast oriented (emergency vehicle notices, forexample).

However, there have also been proposals for communications based onthose used in mobile telecommunications systems, such as Long TermEvolution (LTE) based networks operating on International MobileTelecommunications (IMT) bands, to help support ITS applications, forexample to provide more capacity and potentially provide for wider andcheaper coverage. In particular, where the existing cellular networkalready covers roadways the capital expenditure costs associated withusing cellular mobile telecommunications techniques for ITS applicationsmay be significantly less than what would be needed for setting up a newDSRC-based ITS network.

Accordingly, an Intelligent Transport System may rely on D2Dcommunications of the kind proposed for mobile wirelesstelecommunications systems to allow vehicles to communicate with oneanother and with other terminal devices or network infrastructureequipment, such as a base station or specific road side infrastructure.In this regard, communications associated with connected vehicle systemsmay be conveniently referred to as V2X (vehicle-to-everything)communications, which may comprise V2V (vehicle-to-vehicle), V2P(vehicle-to-pedestrian) and V2I (vehicle-to-infrastructure).Infrastructure in this case may be a roadside ITS related infrastructureelement, which may be referred to as a road side unit (RSU), or aconventional Internet or mobile network infrastructure element. Someexamples or services in connected a vehicle context are CooperativeAwareness Message (CAM) and Decentralised Environmental Notification(DEN). These constitute applications such as allowing emergency vehiclesto broadcast their presence and allowing roadside infrastructure tobroadcast speed limit information to vehicles.

It has been proposed that V2X communications may be implemented usingdedicated Road Side Units (RSUs) which communicate with vehicles andwhich assign radio resources for use by the vehicles in V2Xcommunications. In particular, such RSUs may allocate D2D radioresources for use in V2X communications. Nonetheless, it is also to beexpected that there may be situations where vehicles will wish toautonomously communicate directly with one another (V2V) without anynetwork infrastructure interaction, for example because not all theroadways may have RSUs installed, particularly in more rural areas.

As noted above, one attractive aspect of D2D type communications whichmay be particularly, but not exclusively, well suited to vehicularapplications (i.e. V2V) is the reduction in signalling latency that canbe achieved with D2D communications as compared to conventionalcommunications routed via network infrastructure equipment, such as abase station. This can be helpful in a V2V context, for example, if avehicle wishes to transmit high priority signalling to notify othervehicles of an emergency situation, for example a crash or warning of anemergency services vehicle.

Although D2D type communications can help information to be exchangedbetween terminal devices more quickly than with more conventionaltechniques in which data is routed through a base station or othernetwork infrastructure element, the inventors have recognised therestill remains a need for new approaches which can still further reducethe time taken to exchange data between terminal devices in a wirelesstelecommunications system in a device-to-device (e.g. V2X) mode.

SUMMARY

The present disclosure can help address or mitigate at least some of theissues discussed above.

Respective aspects and features of the present disclosure are defined inthe appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the present technology. The described embodiments,together with further advantages, will be best understood by referenceto the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein likereference numerals designate identical or corresponding parts throughoutthe several views, and wherein:

FIG. 1 provides a schematic diagram illustrating an example of a mobiletelecommunication system;

FIG. 2 schematically represents a wireless telecommunications systemaccording to certain embodiments of the disclosure;

FIG. 3 schematically represents approaches for achieving synchronisationfor device-to-device communications;

FIG. 4 is a ladder diagram schematically representing a mode ofoperation for a wireless telecommunications system in accordance withcertain embodiments of the disclosure; and

FIG. 5 is a ladder diagram schematically representing mode of operationfor a wireless telecommunications system in accordance with certainother embodiments of the disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a mobile (cellular) telecommunications network/system100, in this example operating generally in accordance with LTEprinciples, and which may be adapted to implement embodiments of thedisclosure as described further below. Various elements of FIG. 1 andtheir respective modes of operation are well-known and defined in therelevant standards administered by the 3GPP® body, and also described inmany books on the subject, for example, Holma, H. and Toskala, A. [1].It will be appreciated that operational aspects of thetelecommunications network which are not specifically described belowmay be implemented in accordance with any known techniques, for exampleaccording to the relevant standards and known variations thereof.Furthermore, it will be appreciated that whilst some specific examplesdescribed herein may refer to implementations based around particular3GPP implementations, the same principles can be applied regardless ofthe underlying operating principles of the network. That is to say, thesame principles can be applied for wireless telecommunications networksoperating in accordance with other standards, whether past, current oryet to be specified.

The network 100 includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices104. Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink. Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink. The uplink and downlink communications are made usingradio resources that may be used by the operator of the network 100. Thecore network 102 routes data to and from the terminal devices 104 viathe respective base stations 101 and provides functions such asauthentication, mobility management, charging and so on. In addition tothe base stations 101 and terminal devices 104, the system 100 furthercomprises one or more relay nodes/devices 105. These may be used toenhance coverage for terminal devices operating in the relevant cell(s).The deployment of relay nodes (e.g. in terms of their locations) mayfollow generally established techniques for using relay nodes to supportcoverage in wireless telecommunications systems. In terms ofterminology, it will be appreciated that terminal devices may also bereferred to as mobile stations, user equipment (UE), user terminal,mobile radio, and so forth. Similarly, base stations may also bereferred to as transceiver stations/nodeBs/e-nodeBs, and so forth.Furthermore, relay nodes may also be referred to as relaydevices/relays, and so forth. In some example implementations of thepresent disclosure, a terminal device may be operating as a relay nodeto assist in supporting communications associated with other terminaldevices. That is to say, the functionality of a relay device may beprovided by a suitably configured terminal device.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA).

FIG. 2 schematically shows a telecommunications system 200 supportingdevice-to-device communications in accordance with an embodiment of thedisclosure. In particular, FIG. 2 represents an operating scenario inthe context of an Intelligent Transport System (ITS) scheme wherebyvehicles equipped with terminal devices are configured to supportdevice-to-device communications (vehicle-to-vehicle communications) toallow them to communicate with one another to exchange information usingthe radio resources of the wireless telecommunications system 200. Thetelecommunications system 200 in this example is based broadly on aLTE-type architecture with modifications to support device-to-devicecommunications (i.e. direct signalling exchange between terminal devicesto communicate data between them) generally in accordance withpreviously proposed schemes for D2D communications. As such many aspectsof the operation of the telecommunications system 200 are already knownand understood and not described here in detail in the interest ofbrevity. Operational aspects of the telecommunications system 200 whichare not specifically described herein may be implemented in accordancewith any known techniques, for example according to the establishedLTE-standards and known variations and modifications thereof (e.g. toprovide/introduce support for D2D communications in avehicle-to-vehicle/ITS context).

The telecommunications system 200 comprises a core network part (evolvedpacket core) 202 coupled to a radio network part. The radio network partcomprises a base station (evolved-nodeB) 204, a first terminal device206 and a second terminal device 208. Each terminal device is deployedwithin a vehicle for providing vehicle-to-vehicle communicationfunctionality. It will of course be appreciated that in practice theradio network part will comprise a plurality of base stations serving alarger number of terminal devices (vehicles) across variouscommunication cells. However, only a single base station and twoterminal devices are shown in FIG. 2 in the interests of simplicity forthis figure.

As with a conventional mobile radio network, the terminal devices 206,208 are arranged to communicate data to and from the base station(transceiver station) 204. The base station is in turn communicativelyconnected to a serving gateway, S-GW, (not shown) in the core networkpart which is arranged to perform routing and management of mobilecommunications services to the terminal devices in thetelecommunications system 200 via the base station 204. In order tomaintain mobility management and connectivity, the core network part 202also includes a mobility management entity (not shown) which manages theenhanced packet service, EPS, connections with the terminal devices 206,208 operating in the communications system based on subscriberinformation stored in a home subscriber server, HSS. Other networkcomponents in the core network (also not shown for simplicity) include apolicy charging and resource function, PCRF, and a packet data networkgateway, PDN-GW, which provides a connection from the core network part202 to an external packet data network, for example the Internet. Asnoted above, the operation of the various elements of the communicationssystem 200 shown in FIG. 2 may be broadly conventional apart from wheremodified to provide functionality in accordance with embodiments of thedisclosure as discussed herein. It will further be appreciated that forother implementations which are based around wireless telecommunicationssystems operating in accordance with different standards, the networkarchitecture may be correspondingly different.

The first and second terminal devices 206, 208 are D2D (morespecifically, V2V) enabled devices configured to operate in accordancewith embodiments of the present disclosure as described herein. Theterminal devices 206, 208 each comprise a respective transceiver unit205, 209 for transmission and reception of wireless signals and arespective controller unit 207, 211 configured to control the respectiveterminal devices 206, 208. The respective controller units 207, 211 mayeach comprise a processor unit which is suitably configured/programmedto provide the desired functionality using conventionalprogramming/configuration techniques for equipment in wirelesstelecommunications systems. The respective transceiver units 205, 209and controller units 207, 211 are schematically shown in FIG. 2 asseparate elements. However, it will be appreciated for each of theterminal devices the functionality of the terminal device's receiver andcontroller units can be provided in various different ways, for exampleusing a single suitably programmed general purpose computer, or suitablyconfigured application-specific integrated circuit(s)/circuitry. It willbe appreciated the first and second terminal devices 206, 208 will ingeneral comprise various other elements associated with their operatingfunctionality in accordance with established wireless telecommunicationstechniques (e.g. a power source, possibly a user interface, and soforth).

The base station 204 is configured to support communications with theterminal devices and may also in some situations for some examples playa role in configuring aspects of D2D communications between the terminaldevices, for example establishing which radio resources may be used forD2D communications between terminal devices operating within thecoverage area of the base station 204. The base station 204 comprises atransceiver unit 201 for transmission and reception of wireless signalsand a controller unit 203 configured to control the base station 204.The controller unit 203 may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality usingconventional programming/configuration techniques for equipment inwireless telecommunications systems. The transceiver unit 201 and thecontroller unit 203 are schematically shown in FIG. 2 as separateelements for ease of representation. However, it will be appreciatedthat the functionality of these units can be provided in variousdifferent ways, for example using a single suitably programmed generalpurpose computer, or suitably configured application-specific integratedcircuit(s)/circuitry or using a plurality of discretecircuitry/processing elements for providing different elements of thedesired functionality. It will be appreciated the base station 204 willin general comprise various other elements associated with its operatingfunctionality. For example, the base station 204 will in generalcomprise a scheduling entity responsible for scheduling communications.The functionality of the scheduling entity may, for example, be subsumedby the controller unit 203.

Thus, the base station 204 is configured to communicate data with thefirst terminal device 206 over a first radio communication link 210 andcommunicate data with the second terminal device 208 over a second radiocommunication link 212 when the respective terminal devices are incoverage of the base station. Both radio links may be supported within asingle radio frame structure associated with the base station 204. It isassumed here the base station 204 is configured to communicate with theterminal devices 206, 208 over the respective radio communication links210, 212 generally in accordance with the established principles ofLTE-based communications. Nevertheless, it will be appreciated that someembodiments may be implemented without the respective terminal devicesundertaking any base station communications, and in this regard theprinciples described herein with regards to vehicle-to-vehiclecommunications may for some implementations be undertaken regardless ofwhether the respective vehicles (mobile terminals) are in coverage orout of coverage of a base station.

In addition to the terminal devices 206, 208 being arranged tocommunicate data to and from the base station (transceiver station) 204over the respective first and second radio communication links 210, 212,the terminal devices 206, 208 are further arranged to communicate withone another (and other terminal devices within the wirelesstelecommunications system) in a device-to-device (D2D) manner over a D2Dradio communication link 214, as schematically indicated in the figure.The underlying principles of the D2D communications supported in thewireless telecommunications system of FIG. 2 may follow any previouslyproposed techniques, but with modifications to support approaches inaccordance with embodiments of the disclosure as described herein.

As is well known, synchronisation is an important aspect of wirelesstelecommunications. To support reliable communications in a wirelesstelecommunications system, the entities involved in the communications,i.e. the transmitter and receiver entities, should be appropriatelysynchronised with one another. Wireless communications are typicallyundertaken using a radio frame structure comprising grid of radioresources spanning time (e.g. time slots in a sub-frame) and frequency(e.g. OFDM subcarriers across a system bandwidth). A transmitting entitywill undertake transmissions at particular times and on particularfrequency resources, e.g. determined in accordance with a schedulingprocess. The receiving entity needs to be sufficiently well timesynchronised to the transmitting entity to allow the receiving entity toproperly locate and decode the transmissions on the relevant time andfrequency resources.

The process of synchronisation is well understood in terms of howsynchronisation may be achieved using synchronisation signallingtransmitted by one of the entities. For example, in an LTE-basedwireless telecommunications system, terminal devices will typicallysynchronise to the base station to which they are connected by receivingand processing the primary synchronisation signalling (PSS) andsecondary synchronisation signalling (SSS) transmitted by the basestation.

Within a cell of a wireless telecommunications system it is typical forthe active terminal devices within coverage of a base station servingthe cell to be synchronised to the base station. Because of this it istypical for the active terminal devices within network coverage to besynchronised with one another because they are all respectivelysynchronised to the network. In this regard the base station may in somerespects be considered to provide for centralised synchronisation ofterminal devices. Thus, a first in-coverage terminal device and a secondin coverage terminal device which are to exchange data in adevice-to-device manner will typically be synchronised to one another byvirtue of both being synchronised to the base station. However, in awireless telecommunications system supporting device-to-devicecommunications there can arise a situation in which a first terminaldevice wishes to communicate with a second terminal device in adevice-to-device manner, but one or other (or both) of these terminaldevices are out of coverage of the base station. This means at least oneof the terminal devices will not be able to synchronise to the basestation, and so will instead need to achieve synchronisation with theother terminal device through a different mechanism. To address thisissue, it is proposed to use what may be referred to as sidelinksynchronisation procedures whereby a first terminal device transmitssynchronisation signalling to allow a second terminal device tosynchronise to the first terminal device. In this regard the underlyingprocess of synchronisation, for example in terms of the nature of thesynchronisation signalling, may be performed in accordance with the sameprinciples as for conventional base station synchronisation, the majordifference being the synchronisation signalling is transmitted by aterminal device, rather than a base station.

FIG. 3 illustrates an example of how D2D UEs may achieve synchronisationin accordance with established 3GPP Release 12 D2D proposals. See, forexample, ETSI TS 136 331 v12.7.0 (2015-09)/3GPP TS 36.331 version 12.7.0Release 12 [2] for more details, for example in Section 5.10. FIG. 3shows a base station 204 (e.g. corresponding to the base stationrepresented in FIG. 2) having a coverage area 305, and a first terminaldevice 301, a second terminal device 302 and a third terminal device 303(e.g. such as the terminal devices 206, 208 represented in FIG. 2). Itis assumed here the first terminal device 301 is within the coveragearea 305 of the base station 204, the second terminal device 302 isoutside the coverage area 305 of the base station 204, but within theD2D communication range of the first terminal device 301, and the thirdterminal device 303 is outside the coverage area 305 of the base station204 and outside the D2D communication range of the first terminal device301, but within the D2D communication range of the second terminaldevice 302.

The first terminal device 301 achieves synchronisation with the basestation in the usual way, for example through reception of primary andsecondary synchronisation signalling. The first terminal device 301 alsoreceives from the base station configuration settings for D2Dsynchronisation signalling that the first terminal device 301 maybroadcast to help other devices within its D2D communication range toachieve synchronisation (schematically indicated in FIG. 3 by thesignalling arrow labelled 306). The first terminal device 301 is thusconfigured to broadcast D2D synchronisation signalling (D2DSS) inaccordance with the configuration settings received from the basestation 204 (schematically indicated in FIG. 3 by the signalling arrowlabelled 307). The second terminal device 302 cannot receive the primaryand secondary synchronisation signalling from the base station, but canreceive the D2DSS 307 from the first terminal device 301, which allowsthe second terminal device 302 to synchronise to the first terminaldevice 301, which is in turn synchronised to the base station 204. Thesecond terminal device 302 is in turn configured by signalling receivedfrom the first terminal device 301 to broadcast D2D synchronisationsignalling (D2DSS) in accordance with the configuration settingsoriginally transmitted by the base station 204. The third terminaldevice 303 cannot receive the primary and secondary synchronisationsignalling from the base station nor the D2DSS 307 broadcast by thefirst terminal device 301, but can receives the D2DSS 308 from thesecond terminal device 302, which allows the third terminal device 303to synchronise to the second terminal device 302, which is in turnsynchronised to the first terminal device, which is in turn synchronisedto the base station 204. Thus, a process is provided by which theterminal devices can synchronise with one another. A similar approachcan be adopted when all terminal devices are out of coverage, except thefirst terminal device will define the synchronisation to which the otherterminal devices are synchronised, rather than receive thesynchronisation from the network.

Whilst FIG. 3 represents one particular approach for achieving D2Dsynchronisation, it will be appreciated the specific processes forachieving D2D synchronisation (e.g. in terms of the specific nature ofthe synchronisation signalling and how it is configured among devices)is not fundamentally significant to the principles described herein.Rather, what is significant, and as discussed further below, is that awireless telecommunications system supports multiple synchronisationprocedures, with the specific processes associated with each of theseprocedures not being directly relevant.

Thus, in a wireless telecommunications system supporting D2Dcommunications, different terminal devices may achieve theircommunication synchronisation from different sources. In some respectsthe types of synchronisation source can be categorised into two types. Afirst type is where the fundamental timing reference for synchronisationis derived from a network infrastructure element, such as a basestation, and synchronisation signalling from this type ofsynchronisation source may sometimes be referred to as D2DSSue_net (thesubscript “net” indicating a network derived timing). A second type iswhere the fundamental timing reference for synchronisation is notderived from a network infrastructure element, and synchronisationsignalling from this type of synchronisation source may sometimes bereferred to as D2DSSue_oon (the subscript “oon” indicating an out ofnetwork derived timing).

Thus, a terminal device may achieve synchronisation from a number ofdifferent sources and be configured to prioritise the different sourcesto use for synchronisation. For example, it has been proposed a terminaldevice will prioritise synchronisation sources in the following order.

1) The terminal device will first seek to achieve synchronisation usingsynchronisation signalling received directly from network infrastructureequipment (e.g. base station).2) If the terminal device is not able to achieve synchronisation usingsynchronisation signalling received directly from network infrastructureequipment, the terminal device will next seek to achieve synchronisationusing synchronisation signalling received from a terminal device whichhas achieved synchronisation using synchronisation signalling receiveddirectly from network infrastructure equipment (i.e. an in-coverageterminal device).3) If the terminal device is not able to achieve synchronisation usingsynchronisation signalling received directly from network infrastructureequipment or from an in-coverage terminal device, the terminal devicewill next seek to achieve synchronisation using synchronisationsignalling received from a terminal device which is out of networkcoverage (i.e. a terminal device which has not itself achievesynchronisation using the transaction signalling received directly fromnetwork infrastructure equipment).

The process of selecting a synchronisation source in accordance with aprioritised list of potential sources, for example as discussed above,may be referred to as synchronisation source selection.

The inventors have recognised an issue with existing processes forsynchronisation source selection is the time taken to make the selectioncan give rise to delays which may be undesirable, for example inrelation to high priority/urgent/emergency communications.

FIG. 4 is a ladder diagram schematically representing some modes ofoperation in a wireless telecommunications system according to certainembodiments of the disclosure. The figure schematically represents someoperating aspects and signalling exchange for a base station 204, afirst terminal device 206, and a second terminal device 208 of the kinddiscussed above with reference to FIG. 2. It is assumed for this examplethe first terminal device 206 is outside a coverage area 305 associatedwith the base station 204, whereas the second terminal device 208 is thecoverage area (cell) 305 of the base station 204.

As schematically indicated in step S1, and in accordance with generallyconventional techniques, the base station broadcasts synchronisationsignalling (SS) on an ongoing basis. For example, in an LTE context,synchronisation signalling (e.g. PSS/SSS) is transmitted by the basestation twice in every 10 ms frame.

As schematically indicated in step S2, the first terminal device 206also transmits synchronisation signalling (D2DSS) to help maintainsynchronisation amongst devices operable to communicate with the firstterminal device in a device-to-device manner.

The second terminal device 208 is capable of achieving synchronisationusing synchronisation signalling received from the base station 204(i.e. the synchronisation signalling SS transmitted by the base station204 in step S1) and using synchronisation signalling received from thefirst terminal device 206 (i.e. the synchronisation signalling D2DSStransmitted by the first terminal device 206 in step S2). However, asdiscussed above, the second terminal device 208 is configured duringnormal operation to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the base station oversynchronisation signalling transmitted by the first terminal device.That is to say, the default position for the second terminal device 208is to seek to achieve synchronisation using synchronisation signallingreceived from the base station before seeking to do so using signallingreceived from the first terminal device 206. This is schematicallyrepresented in FIG. 4 by step S3.

Thus, so long as the second terminal device 208 is correctly receivingsynchronisation signalling from the base station 204, the secondterminal device 208 will in accordance with previously proposedtechniques rely on this synchronisation signalling to achievesynchronisation for its communications. However, in accordance withcertain embodiments of the disclosure as discussed herein, the firstterminal device 206 is able to in effect force the second terminaldevice 208 to seek to achieve synchronisation using synchronisationsignalling transmitted by the first terminal device 206 in preference tosynchronisation signalling transmitted by the base station 204. That isto say, in accordance with certain embodiments of the disclosure, afirst terminal device (or other network entity) can control a secondterminal device (or other network entity) to override its defaultsynchronisation source prioritisation procedure, for example in order tomore quickly establish communications in the event of an emergencysituation arising.

In step S4 in FIG. 4, the first terminal device 206 detects that anemergency event has occurred. The nature of the emergency is notsignificant to the principles described herein. In the context of avehicle-to-vehicle deployment, the emergency situation might, forexample, correspond with the first terminal device determining that avehicle with which the terminal device is associated has been involvedin a crash or has undergone heavy braking. It will be appreciated themanner in which the occurrence of the emergency event is detected is notsignificant. For example, in practice it may be expected that a vehicle(or other host device) with which the first terminal device isassociated will initially detect the occurrence of the emergency eventfrom its own sensors, and provide an indication of this to the terminaldevice, for example over a communications interface connection betweenthe terminal device and the vehicle (or other host device) with which itis associated.

As schematically indicated in step S5, throughout the operationrepresented in FIG. 4, the base station continues to transmit itssynchronisation signalling (SS) in accordance with conventionaltechniques. That is to say, step S5 is indicative of an ongoing processand is not linked to the detection of the emergency event in step S4.

In step S6, in response to determining the emergency event has occurredin step S4, the first terminal device 206 determines that it shouldtransmit emergency data relating to the event in a device-to-devicemanner. For example, in the case the emergency event is a crash of avehicle supporting vehicle-to-vehicle communications, the first terminaldevice may determine that it should broadcast a warning message toindicate a crash has occurred at its location. This is with a view toproviding surrounding vehicles, for example following vehicles, withinformation that may allow them to react to the occurrence of theemergency event in an appropriate way, for example by quickly coming toa stop.

To allow the first terminal device to establish communications withother terminal devices as quickly as possible in accordance withembodiments of the disclosure, in step S6 the first terminal device 206transmits synchronisation signalling having a predefined characteristicwhich indicates the synchronisation signalling is high prioritysynchronisation signalling (HP-D2DSS). Apart from being associated withthe predefined characteristic indicating the synchronisation signallingis high priority synchronisation signalling, the high prioritysynchronisation signalling may otherwise correspond with conventionalD2D synchronisation signalling, for example of the kind transmitted bythe first terminal device in step S2.

There are various characteristics that may be associated with thesynchronisation signalling of step S6 to indicate the synchronisationsignalling is high priority synchronisation signalling.

In this example the wireless telecommunications system is configured sothe high priority synchronisation signalling HP-D2DSS is transmitted onradio resources (e.g. in terms of time and frequency resources) whichare reserved for the purpose of transmitting high prioritysynchronisation signalling. The radio resources which are reserved forthe purpose of transmitting high priority synchronisation signalling inaccordance with this example approach may be defined/established in anumber of ways. For example, an indication of the reserved radioresources may be defined in an operating standard of the wirelesstelecommunication system in which an embodiment of the disclosure isimplemented, In another example an indication of the reserved radioresources may be received by a terminal device in signalling receivedfrom another entity operating in the wireless telecommunication system,e.g. a base station or other network infrastructure element or anotherterminal device, having selectable control over the choice of radioresources to be reserved for this purpose, for example in systeminformation, SI, signalling or other control signalling. In yet anotherexample an indication of the reserved radio resources may be defined bya Subscriber Identity Module, SIM, or subscription based configurationfor terminal devices implementing an approach in accordance with theprinciples described herein in accordance with conventional approachesfor establishing configuration settings in wireless telecommunicationssystems. It may be considered appropriate for some implementations forthe reserved radio resources to be fixed and non-selectable within agiven wireless telecommunication system (e.g. the reserved resources maybe defined by standard) so the same resources can be used withindifferent wireless telecommunication systems, for example systemsassociated with different mobile network operators, MNOs, to assistinteroperability. It will be appreciated the reserved resources may beresources in a dedicated spectrum, such as for DSRC or a shared spectrumor the licensed spectrum of respective MNOs, or a combination thereof.

Thus, the first terminal device 206 may be configured to transmit itsroutine synchronisation signalling (e.g. as in step S2) using a firstset of radio resources, for example, in terms of specific times andfrequencies within a radio frame structure associated withdevice-to-device transmissions in the wireless telecommunicationssystem, but may further be configured to transmit high prioritysynchronisation signalling (e.g. as in step S6) using a second,different, set of radio resources within the radio frame structure.Apart from the difference in radio resources used, the routinesynchronisation signalling (as in step S2) and the emergency/highpriority synchronisation signalling (as in step S6) might be the same,for example they may use the same types of synchronisation signalsequences/codes.

In another example, the wireless telecommunications system may beconfigured so the high priority synchronisation signalling HP-D2DSS istransmitted in association with an explicit indication, e.g. a data bitassociated with the synchronisation signalling may be set to apredefined value to indicate the synchronisation signalling is highpriority synchronisation signalling.

In yet another example, the wireless telecommunications system may beconfigured so the high priority synchronisation signalling HP-D2DSS istransmitted with a predefined format that indicates the synchronisationsignalling is high priority. For example, the predefined format maycorrespond with a particular choice of synchronisation signallingsequence/code reserved for use by high priority synchronisationsignalling.

Regardless of the manner by which synchronisation signalling isindicated as high priority synchronisation signalling, i.e. regardlessof the nature of the predefined characteristic which is associated withthe synchronisation signalling transmitted by the first terminal device206 to indicate the synchronisation signalling is high prioritysynchronisation signalling, the other terminal devices operating in thewireless telecommunications system, for example the second terminaldevice 208 represented in FIG. 4, are configured in accordance with anembodiment of the disclosure to monitor for synchronisation signallinghaving/being associated with the predefined characteristic(s).

Thus, in this example in which it is assumed the predefinedcharacteristic corresponds with using particular radio resourcesreserved for high priority synchronisation signalling, the secondterminal device 208 is configured to monitor these radio resources on anongoing basis to detect the presence of synchronisation signalling onthese radio resources. The detection of synchronisation signalling onthese resources by a terminal device thus corresponds with the detectionof high priority synchronisation signalling.

As schematically indicated in step S7, the second terminal devicedetects the high priority synchronisation signalling (i.e. the secondterminal device detects synchronisation signalling has been transmittedin association with the predefined characteristic which indicates thesynchronisation signalling is high priority synchronisation signalling).

In step S8 the second terminal device seeks to synchronise to the highpriority synchronisation signalling received in step S7. The underlyingprocess of synchronisation may be performed in accordance withconventional techniques. One significant aspect of the operation of thesecond terminal device in this regard is that it seeks to synchronise tothe synchronisation signalling received from the terminal device, ratherthan synchronisation signalling received from the base station, which iscontrary to its default synchronisation procedure. That is to say, thesecond terminal device's default synchronisation procedure has beenoverridden by receiving the emergency/high priority signal datasignalling from the first terminal device.

In step S9 the first terminal device 206 generates the high prioritydata regarding the event which it wishes to transmit. For example, inthe case of the emergency event of step S4 corresponding to a crash, thehigh priority data may comprise an indication of the occurrence andlocation of the crash.

In step S10 the first terminal device 206 proceeds to schedule andtransmit the high priority data generated step S9, for example in abroadcast device-to-device manner, for reception by the second terminaldevice 208. This may be performed in accordance with conventionaltechniques. However, in some cases, the resources to be used fortransmitting the high priority may also be pre-defined, for example interms of particular time and frequency resources within adevice-to-device communications frame structure, thereby obviating theneed for the first terminal device to separately schedule thetransmissions, which can further help reduce delays between thedetection of the emergency event and the transmission of the highpriority data relating to the event.

Thus, in accordance with the process set out in FIG. 4, a first terminaldevice detecting the occurrence of an emergency situation and acorresponding desire to transmit data quickly to other terminal devicesto provide them with information regarding the emergency situation canin effect force other terminal devices to synchronise to the firstterminal device in preference to any other synchronisation signallingthey are receiving, and regardless of a default synchronisationprioritisation procedure, thereby allowing the data to be data moretransmitted quickly than if the first terminal device were required tofirst achieved synchronisation.

In some scenarios the first terminal device may not have achievedsynchronisation with the other entities operating in the network at thetime of the emergency event, for example because it is out of coverageof a base station, as schematically represented in FIG. 4. In accordancewith the principles described herein, the first terminal device maynonetheless be configured to proceed to transmit high prioritysynchronisation signalling reflecting its current synchronisation, forexample based on its most-recently achieved synchronisation or aself-created synchronisation. Significantly, the first terminal devicemay do this without going through a synchronisation source selectionprocedure (i.e. without first trying to achieve network synchronisationby trying to detect and decode synchronisation signalling received froma base station), thereby allowing the terminal device to proceed withtransmitting data regarding the occurrence of the event more quicklythan would otherwise be the case. That is to say, with reference to FIG.4, prior to transmitting the high priority synchronisation signalling(HP-D2DSS) in step S6, the first terminal device 206 does not attempt toachieve synchronisation in accordance with its own defaultsynchronisation source selection process.

In general it may be expected that emergency events occur rarely sothere is a relatively small chance of a collision between high prioritysynchronisation signalling transmitted using the same predefinedcharacteristics from two different terminal devices. However, onescenario in which a high priority synchronisation signalling collisionmay be considered more likely to occur is where a crash involves two (ormore) vehicles which are respectively associated with terminal devicesconfigured to operate in accordance with the principles describedherein. In this scenario, different terminal devices associated with thedifferent vehicles involved in the crash may simultaneously undertakethe processing represented in FIG. 4, which may result in a signallingcollision in respect of high priority synchronisation signalling fromthe respective terminal devices.

One way to help reduce the likelihood of this occurring would be fordifferent terminal devices to wait for a selectable delay betweendetecting the emergency event has occurred and transmitting their highpriority signals a signalling. This delay may be quasi-random, forexample, the respective terminal devices may determine their respectivedelays from an identifier for the respective terminal devices. Forexample, the delay may be selected as a number of millisecondscorresponding to the last digit of the terminal device's respectiveidentifiers. Another way would be for terminal devices to select thepredefined characteristic indicating their synchronisation signalling ishigh priority synchronisation signalling from a plurality of potentialpredefined characteristics, for example from a plurality of potentialradio resources, e.g. at different times. The selection of thepredefined characteristic by a particular terminal device may bequasi-random, for example, the respective terminal devices may determinetheir respective predefined characteristics from an identifier for therespective terminal devices. For example, there may be five potentialpredefined characteristics (e.g. five different reservations of radioresources to use for high priority synchronisation signals), and aparticular terminal device may select a particular predefinedcharacteristic according to a remainder value obtained when dividing anumeric identifier for the terminal device by five.

Thus, the above described approaches can help avoid collisions byincreasing the likelihood that one of the terminal devices willbroadcast its emergency synchronisation signalling before the other(rather than at the same time). For example, suppose vehicle Aassociated with terminal device A crashes in to vehicle B associatedwith terminal device B. Further suppose that, in accordance with theprinciples described above, terminal device A happens to transmit itshigh priority synchronisation signalling before terminal device B. Inaddition to waiting to transmit its own synchronisation signalling,terminal device B may, like all other terminal devices, be monitoringfor high priority synchronisation signalling from other terminaldevices. On detecting the high priority synchronisation signalling fromterminal device A, terminal device B may thus assume that all otherterminal devices will synchronise using the high prioritysynchronisation signalling from terminal device A, and so terminaldevice B may do the same, and need not transmit its own high prioritysynchronisation signalling.

Although the examples described above have focused on the situation inwhich a first terminal is communicating with a second terminal device,it will be appreciated in general there may be further terminal devicesperforming corresponding processes to that represented in FIG. 4.Furthermore, it will be appreciated that the roles of the first and thesecond terminal devices in FIG. 4 could be reversed. That is to say, ifit were the second terminal device that detected the occurrence of anemergency event, it may itself transmit high priority synchronisationsignalling that is received by the first terminal device.

FIG. 5 is a ladder diagram schematically representing some modes ofoperation in a wireless telecommunications system according to certainembodiments of the disclosure. FIG. 5 is similar to, and will beunderstood from, FIG. 4. However, whereas FIG. 4 schematicallyrepresents a scenario in which the first terminal device 206 is innetwork coverage (i.e. within the coverage 305 of the base station 204)and the second terminal device 206 is out of network coverage (i.e.outside the coverage 305 of the base station 204), FIG. 5 schematicallyrepresents a scenario in which the first terminal device 206 and thesecond terminal device, as well as a third terminal device 2014, areinvolved in D2D communications while all are out of network coverage.

Step S2, S4 and S6 to S10 in the example of FIG. 5 correspond directlywith the correspondingly numbered steps in FIG. 4 and are not describedagain in the interest of brevity. However, steps S1, S3 and S5 in FIG. 4(i.e. the steps associated with the synchronisation signalling SSreceived from the base station) are not present in FIG. 5 (because thesecond terminal device 206 is assumed to be out of coverage in FIG. 5).Instead, the scenario of FIG. 5 reflects a situation on which there isanother terminal device (i.e. the third terminal device 214) which is inD2D communication range of the second terminal device and transmittingits own D2D synchronisation signalling.

Thus in step T1 (which broadly corresponds with step S1 in FIG. 4 exceptin terms of the entity making the transmission), and in accordance withpreviously proposed techniques for D2D synchronisation procedures, e.g.as described in ETSI TS 136 331 v12.7.0 (2015-09)/3GPP TS 36.331 version12.7.0 Release 12 [2], the third terminal device 214 broadcasts D2Dsynchronisation signalling (D2DSS) on an ongoing basis.

As schematically indicated in FIG. 5 step S2, the first terminal device206 also transmits synchronisation signalling (D2DSS) to help maintainsynchronisation amongst devices operable to communicate with the firstterminal device in a device-to-device manner. This step correspondswith, and will be understood from, step S2 in FIG. 4 discussed above.

It is assumed here that for the second terminal device 208, the thirdterminal device 214 is a more preferred synchronisation source than thefirst terminal device 206 according to the relevant synchronisationsource selection procedures. This may be, for example, because the thirdterminal device 214 has achieved synchronisation using signalling from aterminal device which is itself in network coverage (not shown in FIG.5), whereas the first terminal device 206 has not.

The second terminal device 208 is capable of achieving synchronisationusing synchronisation signalling received from the third terminal device214 (i.e. the synchronisation signalling D2DSS transmitted by the thirdterminal device 214 in step T1) and using synchronisation signallingreceived from the first terminal device 206 (i.e. the synchronisationsignalling D2DSS transmitted by the first terminal device 206 in stepS2). However, as discussed above, it is assumed for the scenariorepresented in FIG. 5 that for the second terminal device 208, the thirdterminal device 214 is a more preferred synchronisation source than thefirst terminal device 206. That is to say, the current default positionfor the second terminal device 208 is to seek to achieve synchronisationusing synchronisation signalling received from the third terminal device214 before seeking to do so using signalling received from the firstterminal device 206. This is schematically represented in FIG. 5 by stepT3.

Thus, so long as the second terminal device 208 is correctly receivingsynchronisation signalling from the third terminal device 214, thesecond terminal device 208 will in accordance with previously proposedtechniques rely on this synchronisation signalling to achievesynchronisation for its communications. However, in accordance withcertain embodiments of the disclosure as discussed herein, and in amanner similar to that described above with reference to FIG. 4, thefirst terminal device 206 is able to in effect force the second terminaldevice 208 to seek to achieve synchronisation using synchronisationsignalling transmitted by the first terminal device 206 in preference tosynchronisation signalling transmitted by the third terminal device 214.

In step S4 of FIG. 5, the first terminal device 206 detects that anemergency event has occurred. This step corresponds with, and will beunderstood from, step S4 in FIG. 4 discussed above.

As schematically indicated in step T5, throughout the operationrepresented in FIG. 5, the third terminal device 214 continues totransmit its synchronisation signalling (D2DSS) in accordance withconventional techniques. That is to say, step S5 is indicative of anongoing process and is not linked to the detection of the emergencyevent in step S4.

In step S6 of FIG. 5, in response to determining the emergency event hasoccurred in step S4, the first terminal device 206 determines that itshould transmit emergency data relating to the event in adevice-to-device manner. This step corresponds with, and will beunderstood from, step S6 in FIG. 4.

As schematically indicated in step S7 of FIG. 5, the second terminaldevice detects the high priority synchronisation signalling (i.e. thesecond terminal device detects synchronisation signalling has beentransmitted in association with the predefined characteristic whichindicates the synchronisation signalling is high prioritysynchronisation signalling). This step corresponds with, and will beunderstood from, step S7 in FIG. 4 discussed above.

In step S8 of FIG. 5 the second terminal device seeks to synchronise tothe high priority synchronisation signalling received in step S7. Thisstep corresponds with, and will be understood from, step S8 in FIG. 4discussed above.

In step S9 of FIG. 5 the first terminal device 206 generates the highpriority data regarding the event which it wishes to transmit. This stepcorresponds with, and will be understood from, step S9 in FIG. 4discussed above.

In step S10 of FIG. 5 the first terminal device 206 proceeds to scheduleand transmit the high priority data generated step S9. This stepcorresponds with, and will be understood from, step S9 in FIG. 4discussed above.

It will be appreciated the specific information content of a highpriority communication associated with high priority synchronisationsignalling between vehicles is not significant to the principlesunderlying the operations described herein. Thus in any given case theinformation content will depend on the implementation at hand and thefunctionality the particular ITS scheme provides. For example, in someimplementations the information exchanged through the vehicle-to-vehiclecommunications may comprise information regarding the occurrence of aroad traffic accident, warnings relating to road conditions, for examplea detected high risk of skidding, or information regarding relativelyunusual aspect of a vehicle's operation, for example sudden heavybraking. What is significant in accordance with the principles describedherein, is the manner in which communications may be undertaken moreswiftly than existing schemes, and not the information content of thecommunications themselves or the trigger events giving rise to thecommunications.

It will be appreciated that while some of the above-describedembodiments have focused on examples in which a base station of thewireless telecommunications system is providing functionality inaccordance with the principles described herein, in otherimplementations similar functionality may be provided by othercomponents of the wireless telecommunications network infrastructure.For example, some, or all, of the processing described above in relationto the base station may be provided by a core network component of thewireless telecommunications system and/or similar functionality may beprovided by other infrastructure elements, such as relay nodes and/ordedicated units for supporting an ITS scheme, for example roadside units(RSUs) deployed in association with a road network to facilitatevehicle-to-vehicle communications in accordance with previously proposedschemes. In this regard a base station may be considered as one exampleof network infrastructure equipment and maybe configured to providefunctionality of the kind described herein.

Thus there has been described a wireless telecommunications systemcomprising a first terminal device and a second terminal device operableto communicate with one another in a synchronised device-to-devicemanner. The second terminal device is operable to selectively achievesynchronisation for receiving data transmissions from the first terminaldevice using synchronisation signalling transmitted by the firstterminal device (direct synchronisation) or synchronisation signallingtransmitted by another network entity, such as a base station (indirectsynchronisation). The second terminal device is configured topreferentially seek to achieve indirect synchronisation over directsynchronisation, unless the second terminal device detectssynchronisation signalling transmitted by the first terminal device isassociated with a predefined characteristic which indicates thesynchronisation signalling transmitted by the first terminal device ishigh priority synchronisation signalling, in which case the secondterminal device is configured to preferentially seek to achievesynchronisation using the high priority direct synchronisationsignalling from the first terminal device over indirect synchronisationsignalling transmitted by the other network entity. This can help thesecond terminal device achieve synchronisation for receiving datatransmissions from the first terminal device more quickly in a highpriority situation.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, define, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

Respective features of the present disclosure are defined by thefollowing numbered paragraphs:

1. A wireless telecommunications system comprising a first terminaldevice and a second terminal device operable to communicate with oneanother in a synchronised device-to-device manner, wherein the secondterminal device is operable to selectively achieve synchronisation forcommunicating with the first terminal device using synchronisationsignalling transmitted by the first terminal device or synchronisationsignalling transmitted by another network entity operating in thewireless telecommunications system, wherein the second terminal deviceis configured to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the other network entity oversynchronisation signalling transmitted by the first terminal device,unless the second terminal device detects synchronisation signallingtransmitted by the first terminal device is associated with a predefinedcharacteristic which indicates the synchronisation signallingtransmitted by the first terminal device is high prioritysynchronisation signalling, in which case the second terminal device isconfigured to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the first terminal device oversynchronisation signalling transmitted by the other network entity.2. The wireless telecommunications system of paragraph 1, wherein thepredefined characteristic which indicates the synchronisation signallingis high priority synchronisation signalling is the use of predefinedradio resources for transmitting the synchronisation signalling.3. The wireless telecommunications system of paragraph 2, wherein thepredefined radio resources are defined by an operating standard of thewireless telecommunication system and/or the first and/or secondterminal devices are operation to receive an indication of thepredefined radio resources from configuration signalling received fromanother entity operating in the wireless telecommunication system and/orfrom a Subscriber Identity Module, SIM, configuration setting and/orfrom a subscription-based configuration setting.4. The wireless telecommunications system of any one of paragraphs 1 to3, wherein the predefined characteristic which indicates thesynchronisation signalling is high priority synchronisation signallingis the presence of a predefined indicator flag transmitted inassociation with the synchronisation signalling.5. The wireless telecommunications system of any one of paragraphs 1 to4, wherein the predefined characteristic which indicates thesynchronisation signalling is high priority synchronisation signallingis the use of predefined format for the synchronisation signalling.6. The wireless telecommunications system of any one of paragraphs 1 to5, wherein the first terminal device is configured to transmit highpriority synchronisation signalling on determining an event has occurredin response to which the first terminal device is configured to transmithigh priority data relating to the event in a device-to-device manner.7. The wireless telecommunications system of paragraph 6, wherein thefirst terminal device is configured to transmit the high prioritysynchronisation signalling without first seeking to synchronised toanother network entity.8. The wireless telecommunications system of paragraph 6 or 7, whereinthe first terminal device is further configured to transmit the highpriority data relating to the event.9. The wireless telecommunications system of paragraph 8, wherein thehigh priority data relating to the event is transmitted by the firstterminal in a device-to-device broadcast manner.10. The wireless telecommunications system of paragraph 8 or 9, whereinthe first terminal device is configured to wait for a selectable delaybefore transmitting the high priority synchronisation signalling afterdetecting the event has occurred for a selectable delay and/or whereinthe first terminal device is configured to select the predefinedcharacteristic for transmitting the high priority synchronisationsignalling from a plurality of predefined characteristics.11. The wireless telecommunications system of paragraph 10, wherein thefirst terminal device is configured to select the delay and/or thepredefined characteristic in a quasi-random manner.12. The wireless telecommunications system of paragraph 11, wherein thefirst terminal device is configured to select the delay and/or thepredefined characteristic by taking account of a terminal deviceidentifier associated with the first terminal device.13. The wireless telecommunications system of any one of paragraphs 1 to12, wherein the second terminal device is configured to monitor forsynchronisation signalling transmitted with the predefinedcharacteristic on an ongoing basis.14. The wireless telecommunications system of any one of paragraphs 1 to13, wherein the first terminal device is operable to selectively achievesynchronisation for communicating with the second terminal device usingsynchronisation signalling transmitted by the second terminal device orsynchronisation signalling transmitted by the other network entity,wherein the first terminal device is configured to preferentially seekto achieve synchronisation using synchronisation signalling transmittedby the other network entity over synchronisation signalling transmittedby the second terminal device, unless the first terminal device detectssynchronisation signalling transmitted by the second terminal device isassociated with a predefined characteristic which indicates thesynchronisation signalling transmitted by the second terminal device ishigh priority synchronisation signalling, in which case the firstterminal device is configured to preferentially seek to achievesynchronisation using synchronisation signalling transmitted by thesecond terminal device over synchronisation signalling transmitted bythe other network entity15. The wireless telecommunications system of any one of paragraphs 1 to14, wherein the wireless telecommunications system further comprises oneor more further terminal devices operable to communicate with the secondterminal device in a synchronised device-to-device manner, wherein thesecond terminal device is operable to selectively achievesynchronisation for receiving data from one of the further terminaldevices using synchronisation signalling transmitted by the one of thefurther terminal devices or synchronisation signalling transmitted byanother network entity operating in the wireless telecommunicationssystem, wherein the second terminal device is configured topreferentially seek to achieve synchronisation using synchronisationsignalling transmitted by the other network entity over synchronisationsignalling transmitted by the one of the further terminal devices,unless the second terminal device detects synchronisation signallingtransmitted by the one of the further terminal devices is associatedwith a predefined characteristic which indicates the synchronisationsignalling is high priority synchronisation signalling, in which casethe second terminal device is configured to preferentially seek toachieve synchronisation using the high priority synchronisationsignalling over synchronisation signalling transmitted by the othernetwork entity.16. The wireless telecommunications system of any one of paragraphs 1 to15, wherein the other network entity comprises network infrastructureequipment or wherein the other network entity comprises another terminaldevice operating in the wireless telecommunications system.17. A method of operating a wireless telecommunications systemcomprising a first terminal device and a second terminal device operableto communicate with one another in a synchronised device-to-devicemanner, wherein the second terminal device is operable to selectivelyachieve synchronisation for communicating with the first terminal deviceusing synchronisation signalling transmitted by the first terminaldevice or synchronisation signalling transmitted by another networkentity operating in the wireless telecommunications system, wherein themethod comprises the second terminal device preferentially seeking toachieve synchronisation using synchronisation signalling transmitted bythe other network entity over synchronisation signalling transmitted bythe first terminal device, unless the second terminal device detectssynchronisation signalling transmitted by the first terminal device isassociated with a predefined characteristic which indicates thesynchronisation signalling transmitted by the first terminal device ishigh priority synchronisation signalling, in which case the secondterminal device preferentially seeks to achieve synchronisation usingthe high priority synchronisation signalling rather than synchronisationsignalling transmitted by the other network entity.18. A first terminal device for use in a wireless telecommunicationssystem comprising the first terminal device and a second terminaldevice, wherein the first terminal device comprises a controller unitand a transceiver unit configured such that the first terminal device isoperable to transmit data to the second terminal device in asynchronised device-to-device manner, wherein the second terminal deviceis operable to achieve synchronisation for receiving data from the firstterminal device using synchronisation signalling transmitted by thefirst terminal device or synchronisation signalling transmitted byanother network entity operating in the wireless telecommunicationssystem, wherein the first terminal device is configured to selectivelytransmit synchronisation signalling with a predefined characteristic toindicate the synchronisation signalling transmitted by the firstterminal device is high priority synchronisation signalling and thesecond terminal device should seek to achieve synchronisation with thefirst terminal device using the high priority synchronisation signallingrather than synchronisation signalling transmitted by the other networkentity.19. A method of operating a first terminal device in a wirelesstelecommunications system comprising the first terminal device and asecond terminal device, wherein the first terminal device is operable totransmit data to the second terminal device in a synchroniseddevice-to-device manner and the second terminal device is operable toachieve synchronisation for receiving data from the first terminaldevice using synchronisation signalling transmitted by the firstterminal device or synchronisation signalling transmitted by anothernetwork entity operating in the wireless telecommunications system,wherein the method comprises the first terminal device selectivelytransmitting synchronisation signalling with a predefined characteristicto indicate the synchronisation signalling transmitted by the firstterminal device is high priority synchronisation signalling and thesecond terminal device should seek to achieve synchronisation with thefirst terminal device using the high priority synchronisation signallingrather than synchronisation signalling transmitted by the other networkentity.20. Integrated circuitry for a first terminal device for use in awireless telecommunications system comprising the first terminal deviceand a second terminal device, wherein the integrated circuitry comprisesa controller element and a transceiver element configured to operatetogether such that the first terminal device is operable to transmitdata to the second terminal device in a synchronised device-to-devicemanner, wherein the second terminal device is operable to achievesynchronisation for receiving data from the first terminal device usingsynchronisation signalling transmitted by the first terminal device orsynchronisation signalling transmitted by another network entityoperating in the wireless telecommunications system, wherein the firstterminal device is configured to selectively transmit synchronisationsignalling with a predefined characteristic to indicate thesynchronisation signalling transmitted by the first terminal device ishigh priority synchronisation signalling and the second terminal deviceshould seek to achieve synchronisation with the first terminal deviceusing the high priority synchronisation signalling rather thansynchronisation signalling transmitted by the other network entity.21. A second terminal device for use in a wireless telecommunicationssystem comprising a first terminal device and the second terminaldevice, wherein the second terminal device comprises a controller unitand a transceiver unit configured such that the second terminal deviceis operable to receive data from the first terminal device in asynchronised device-to-device manner, and wherein the second terminaldevice is operable to selectively achieve synchronisation for receivingdata from the first terminal device using synchronisation signallingreceived from the first terminal device or synchronisation signallingreceived from another network entity operating in the wirelesstelecommunications system, wherein the second terminal device isconfigured to preferentially seek to achieve synchronisation usingsynchronisation signalling received from the other network entity oversynchronisation signalling received from the first terminal device,unless the second terminal device detects synchronisation signallingreceived from the first terminal device is associated with a predefinedcharacteristic which indicates the synchronisation signalling receivedfrom the first terminal device is high priority synchronisationsignalling, in which case the second terminal device is configured toseek to achieve synchronisation using the high priority synchronisationsignalling received from the first terminal device rather thansynchronisation signalling received from the other network entity.22. A method of operating a second terminal device in a wirelesstelecommunications system comprising a first terminal device and thesecond terminal device, wherein the second terminal device is operableto receive data from the first terminal device in a synchroniseddevice-to-device manner, wherein the second terminal device is operableto selectively achieve synchronisation for receiving data from the firstterminal device using synchronisation signalling received from the firstterminal device or synchronisation signalling received from anothernetwork entity operating in the wireless telecommunications system,wherein the second terminal device is configured to preferentially seekto achieve synchronisation using synchronisation signalling receivedfrom the other network entity over synchronisation signalling receivedfrom the first terminal device, unless the second terminal devicedetects synchronisation signalling received from the first terminaldevice is associated with a predefined characteristic which indicatesthe synchronisation signalling received from the first terminal deviceis high priority synchronisation signalling, wherein the methodcomprises the second terminal device detecting synchronisationsignalling received from the first terminal device in association withthe predefined characteristic indicating the synchronisation signallingis high priority synchronisation signalling, and in response thereto,seeking to achieve synchronisation using the high prioritysynchronisation signalling received from the first terminal device inpreference to synchronisation signalling received from the other networkentity.23. Integrated circuitry for a second terminal device for use in awireless telecommunications system comprising a first terminal deviceand the second terminal device, wherein the integrated circuitrycomprises a controller element and a transceiver element configured tooperate together such that the second terminal device is operable toreceive data from the first terminal device in a synchroniseddevice-to-device manner, wherein the second terminal device is operableto selectively achieve synchronisation for receiving data from the firstterminal device using synchronisation signalling received from the firstterminal device or synchronisation signalling received from anothernetwork entity operating in the wireless telecommunications system,wherein the second terminal device is configured to preferentially seekto achieve synchronisation using synchronisation signalling receivedfrom the other network entity over synchronisation signalling receivedfrom the first terminal device, unless the second terminal devicedetects synchronisation signalling received from the first terminaldevice is associated with a predefined characteristic which indicatesthe synchronisation signalling received from the first terminal deviceis high priority synchronisation signalling, wherein the methodcomprises the second terminal device detecting synchronisationsignalling received from the first terminal device in association withthe predefined characteristic indicating the synchronisation signallingis high priority synchronisation signalling, and in response thereto,seeking to achieve synchronisation using the high prioritysynchronisation signalling received from the first terminal device inpreference to synchronisation signalling received from the other networkentity.

REFERENCES

-   [1] Holma H. and Toskala A., “LTE for UMTS OFDMA and SC-FDMA based    radio access”, John Wiley and Sons, 2009-   [2] ETSI TS 136 331 v12.7.0 (2015-09)/3GPP TS 36.331 version 12.7.0    Release 12

1: A wireless telecommunications system comprising a first terminaldevice and a second terminal device operable to communicate with oneanother in a synchronised device-to-device manner, wherein the secondterminal device is operable to selectively achieve synchronisation forcommunicating with the first terminal device using synchronisationsignalling transmitted by the first terminal device or synchronisationsignalling transmitted by another network entity operating in thewireless telecommunications system, wherein the second terminal deviceis configured to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the other network entity oversynchronisation signalling transmitted by the first terminal device,unless the second terminal device detects synchronisation signallingtransmitted by the first terminal device is associated with a predefinedcharacteristic which indicates the synchronisation signallingtransmitted by the first terminal device is high prioritysynchronisation signalling, in which case the second terminal device isconfigured to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the first terminal device oversynchronisation signalling transmitted by the other network entity. 2:The wireless telecommunications system of claim 1, wherein thepredefined characteristic which indicates the synchronisation signallingis high priority synchronisation signalling is the use of predefinedradio resources for transmitting the synchronisation signalling. 3: Thewireless telecommunications system of claim 2, wherein the predefinedradio resources are defined by an operating standard of the wirelesstelecommunication system and/or the first and/or second terminal devicesare operable to receive an indication of the predefined radio resourcesfrom configuration signalling received from another entity operating inthe wireless telecommunication system and/or from a Subscriber IdentityModule, SIM, configuration setting and/or from a subscription-basedconfiguration setting. 4: The wireless telecommunications system ofclaim 1, wherein the predefined characteristic which indicates thesynchronisation signalling is high priority synchronisation signallingis the presence of a predefined indicator flag transmitted inassociation with the synchronisation signalling. 5: The wirelesstelecommunications system of claim 1, wherein the predefinedcharacteristic which indicates the synchronisation signalling is highpriority synchronisation signalling is the use of predefined format forthe synchronisation signalling. 6: The wireless telecommunicationssystem of claim 1, wherein the first terminal device is configured totransmit high priority synchronisation signalling on determining anevent has occurred in response to which the first terminal device isconfigured to transmit high priority data relating to the event in adevice-to-device manner. 7: The wireless telecommunications system ofclaim 6, wherein the first terminal device is configured to transmit thehigh priority synchronisation signalling without first seeking tosynchronised to another network entity. 8: The wirelesstelecommunications system of claim 6, wherein the first terminal deviceis further configured to transmit the high priority data relating to theevent. 9: The wireless telecommunications system of claim 8, wherein thehigh priority data relating to the event is transmitted by the firstterminal in a device-to-device broadcast manner. 10: The wirelesstelecommunications system of claim 8, wherein the first terminal deviceis configured to wait for a selectable delay before transmitting thehigh priority synchronisation signalling after detecting the event hasoccurred for a selectable delay and/or wherein the first terminal deviceis configured to select the predefined characteristic for transmittingthe high priority synchronisation signalling from a plurality ofpredefined characteristics. 11: The wireless telecommunications systemof claim 10, wherein the first terminal device is configured to selectthe delay and/or the predefined characteristic in a quasi-random manner.12: The wireless telecommunications system of claim 11, wherein thefirst terminal device is configured to select the delay and/or thepredefined characteristic by taking account of a terminal deviceidentifier associated with the first terminal device. 13: The wirelesstelecommunications system of claim 1, wherein the second terminal deviceis configured to monitor for synchronisation signalling transmitted withthe predefined characteristic on an ongoing basis. 14: The wirelesstelecommunications system of claim 1, wherein the first terminal deviceis operable to selectively achieve synchronisation for communicatingwith the second terminal device using synchronisation signallingtransmitted by the second terminal device or synchronisation signallingtransmitted by the other network entity, wherein the first terminaldevice is configured to preferentially seek to achieve synchronisationusing synchronisation signalling transmitted by the other network entityover synchronisation signalling transmitted by the second terminaldevice, unless the first terminal device detects synchronisationsignalling transmitted by the second terminal device is associated witha predefined characteristic which indicates the synchronisationsignalling transmitted by the second terminal device is high prioritysynchronisation signalling, in which case the first terminal device isconfigured to preferentially seek to achieve synchronisation usingsynchronisation signalling transmitted by the second terminal deviceover synchronisation signalling transmitted by the other network entity15: The wireless telecommunications system of claim 1, wherein thewireless telecommunications system further comprises one or more furtherterminal devices operable to communicate with the second terminal devicein a synchronised device-to-device manner, wherein the second terminaldevice is operable to selectively achieve synchronisation for receivingdata from one of the further terminal devices using synchronisationsignalling transmitted by the one of the further terminal devices orsynchronisation signalling transmitted by another network entityoperating in the wireless telecommunications system, wherein the secondterminal device is configured to preferentially seek to achievesynchronisation using synchronisation signalling transmitted by theother network entity over synchronisation signalling transmitted by theone of the further terminal devices, unless the second terminal devicedetects synchronisation signalling transmitted by the one of the furtherterminal devices is associated with a predefined characteristic whichindicates the synchronisation signalling is high prioritysynchronisation signalling, in which case the second terminal device isconfigured to preferentially seek to achieve synchronisation using thehigh priority synchronisation signalling over synchronisation signallingtransmitted by the other network entity. 16: The wirelesstelecommunications system of claim 1, wherein the other network entitycomprises network infrastructure equipment or wherein the other networkentity comprises another terminal device operating in the wirelesstelecommunications system. 17: A method of operating a wirelesstelecommunications system comprising a first terminal device and asecond terminal device operable to communicate with one another in asynchronised device-to-device manner, wherein the second terminal deviceis operable to selectively achieve synchronisation for communicatingwith the first terminal device using synchronisation signallingtransmitted by the first terminal device or synchronisation signallingtransmitted by another network entity operating in the wirelesstelecommunications system, wherein the method comprises the secondterminal device preferentially seeking to achieve synchronisation usingsynchronisation signalling transmitted by the other network entity oversynchronisation signalling transmitted by the first terminal device,unless the second terminal device detects synchronisation signallingtransmitted by the first terminal device is associated with a predefinedcharacteristic which indicates the synchronisation signallingtransmitted by the first terminal device is high prioritysynchronisation signalling, in which case the second terminal devicepreferentially seeks to achieve synchronisation using the high prioritysynchronisation signalling rather than synchronisation signallingtransmitted by the other network entity. 18: A first terminal device foruse in a wireless telecommunications system comprising the firstterminal device and a second terminal device, wherein the first terminaldevice comprises a controller unit and a transceiver unit configuredsuch that the first terminal device is operable to transmit data to thesecond terminal device in a synchronised device-to-device manner,wherein the second terminal device is operable to achievesynchronisation for receiving data from the first terminal device usingsynchronisation signalling transmitted by the first terminal device orsynchronisation signalling transmitted by another network entityoperating in the wireless telecommunications system, wherein the firstterminal device is configured to selectively transmit synchronisationsignalling with a predefined characteristic to indicate thesynchronisation signalling transmitted by the first terminal device ishigh priority synchronisation signalling and the second terminal deviceshould seek to achieve synchronisation with the first terminal deviceusing the high priority synchronisation signalling rather thansynchronisation signalling transmitted by the other network entity. 19:A method of operating a first terminal device in a wirelesstelecommunications system comprising the first terminal device and asecond terminal device, wherein the first terminal device is operable totransmit data to the second terminal device in a synchroniseddevice-to-device manner and the second terminal device is operable toachieve synchronisation for receiving data from the first terminaldevice using synchronisation signalling transmitted by the firstterminal device or synchronisation signalling transmitted by anothernetwork entity operating in the wireless telecommunications system,wherein the method comprises the first terminal device selectivelytransmitting synchronisation signalling with a predefined characteristicto indicate the synchronisation signalling transmitted by the firstterminal device is high priority synchronisation signalling and thesecond terminal device should seek to achieve synchronisation with thefirst terminal device using the high priority synchronisation signallingrather than synchronisation signalling transmitted by the other networkentity. 20-21. (canceled) 22: A method of operating a second terminaldevice in a wireless telecommunications system comprising a firstterminal device and the second terminal device, wherein the secondterminal device is operable to receive data from the first terminaldevice in a synchronised device-to-device manner, wherein the secondterminal device is operable to selectively achieve synchronisation forreceiving data from the first terminal device using synchronisationsignalling received from the first terminal device or synchronisationsignalling received from another network entity operating in thewireless telecommunications system, wherein the second terminal deviceis configured to preferentially seek to achieve synchronisation usingsynchronisation signalling received from the other network entity oversynchronisation signalling received from the first terminal device,unless the second terminal device detects synchronisation signallingreceived from the first terminal device is associated with a predefinedcharacteristic which indicates the synchronisation signalling receivedfrom the first terminal device is high priority synchronisationsignalling, wherein the method comprises the second terminal devicedetecting synchronisation signalling received from the first terminaldevice in association with the predefined characteristic indicating thesynchronisation signalling is high priority synchronisation signalling,and in response thereto, seeking to achieve synchronisation using thehigh priority synchronisation signalling received from the firstterminal device in preference to synchronisation signalling receivedfrom the other network entity.
 23. (canceled)